We have studied factors influencing specific binding of low density serum lipoproteins (LDL) to normal and type II homozygous hyperlipoproteinemia skin fibroblasts as well as conditions affecting sterol synthesis in human and animal fibroblast cells in culture. Binding requires protein synthesis since it is reduced by cycloheximide. Binding is also subject to certain hormonal control. Type II condition is not simply a deficiency in binding but it affects LDL intracellular transport. The correlation between cellular proliferation and the role of lipid synthesis has been investigated. The effect of factors stimulating intracellular concentration of cyclic AMP on sterol synthesis has also been studied.